Photonics SpectraOct 2018
A newly fabricated whispering gallery resonator (WGR), in the form of a tiny glass bubble just a hair’s breadth wide, has demonstrated the ability to detect the presence of minute particles more effectively than existing WGRs. The hollow glass microbubbles used for the new sensing regime allow it to move beyond limitations of the evanescent field to achieve greater detection sensitivity.

Using a process akin to traditional glass blowing, researchers at Okinawa Institute of Science and Technology (OIST) Graduate University heated a small glass tube with a laser and had air blown down the tube, creating a spherical chamber. The size of the glass bubble, which can be as small as 100 μ, makes it fragile to handle, but also malleable.

Glass whispering gallery resonator (WGR). The bubble is extremely small, less than the width of a human hair. Courtesy of OIST.
Working from theoretical models, the team showed that theoretically it could increase the size of the light field by using the thin spherical shell, or bubble, rather than a solid sphere.

“We knew we had the techniques and the materials to fabricate the resonator,” said research team leader Jonathan Ward. “Next we had to demonstrate that it could outperform the current types [of WGRs] used for particle detection.”

To test the concept, the team filled the bubble design with a liquid solution containing tiny particles of polystyrene, and shined light along a glass filament to generate a light field in the bubble’s liquid interior. As particles passed within range of the light field, they produced noticeable shifts in the wavelength that were much more pronounced than those seen with a standard spherical WGR.

Test particles (green) are passed through a light field that distorts the light wavelength, which can be used to detect the particles. Courtesy of OIST.The team measured the detection sensitivity in terms of mode shift and broadening, and observed mode shifts of 400 MHz for 100-nm polystyrene particles. In terms of the number of linewidths, this result is 276 times larger than similar experiments with microsphere WGRs, thus showing a significant increase in detection sensitivity beyond the capability of standard evanescent field sensing with WGRs.

Jonathan Ward (left) with research co-author Fuchuan Lei. Courtesy of OIST.Despite their fragility, the glass bubble WGRs are easy to manufacture and can be safely transported in custom-made cases. Ease of manufacturing and transport could allow the sensors to be used in a range of applications, such as testing for toxic molecules in water to detect pollution or detecting blood-borne viruses in low-resource areas where health care may be limited.

Ward and his team anticipate further improvement to the microbubble resonators.

“We’re always pushing to get even more sensitivity and find the smallest particle this sensor can detect,” he said. “We want to push our detection to the physical limits.”